The present invention relates to a purifying agent for purifying soil or ground water, a process for producing the purifying agent, and a method for purifying soil or ground water using the purifying agent. More particularly, the present invention relates to a purifying agent for purifying soil or ground water which can decompose aliphatic organohalogen compounds such as dichloromethane, carbon tetrachloride, 1,2-dichloroethane, 1,1-dichloroethylene, cis-1,2-dichloroethylene, 1,1,1-trichloroethane, 1,1,2-trichloroethane, trichloroethylene, tetrachloroethylene and 1,3-dichloropropene, and aromatic organohalogen compounds such as dioxins and PCB, which are contained in soil or ground water, in an efficient, continuous and economical manner; a process for producing the purifying agent; and a method for purifying the soil or ground water using the purifying agent.
The above aliphatic organohalogen compounds such as trichloroethylene and tetrachlordethylene have been extensively used for cleaning in semiconductor-manufacturing factories and for degreasing metals to be machined.
On the other hand, waste gases, fly ashes and main ashes discharged from incineration furnaces for combusting municipal garbage or industrial wastes, contain aromatic organohalogen compounds such as dioxins having an extremely high toxicity to human bodies even in a trace amount. The waste gases or fly ashes continuously stay around the incineration furnace, so that the dioxins still remain in soil of surrounding regions. Meanwhile, the “dioxins” are a generic name of such compounds formed by replacing hydrogen atoms of dibenzo-p-dioxine, dibenzofuran, etc., with chlorine atoms.
Further, PCB (polychlorinated biphenyl) as the aromatic organohalogen compound has been used in many application as insulating oils for transformers and capacitors, plasticizers or heating medium because of high chemical and thermal stability and excellent electrical insulating property thereof. Since the PCB is very harmful to human bodies, the production and use thereof has been presently prohibited. However, any effective PCB-treating method has not conventionally been established until now and, therefore, a large part of the PCB past used has still been stored without treatment or disposal.
The organohalogen compounds such as aliphatic organohalogen compounds and aromatic organohalogen compounds are hardly decomposable and besides exhibit carcinogenesis as well as a strong toxicity. Therefore, there arises such a significant environmental problem that soil or ground water is contaminated with these organohalogen compounds.
More specifically, upon discharge of the above organohalogen compounds, soil is contaminated with the hardly-decomposable organohalogen compounds, thereby further causing contamination of ground water by the organohalogen compounds. In addition, the contaminated ground water flows out from the contaminated soil and spreads over the surrounding regions, so that the problem of pollution by the organohalogen compounds is caused over wider areas.
The soil is once contaminated with the organohalogen compounds, land involving such soil cannot be reused and developed again. Therefore, there have been proposed various purification methods of purifying the soil and ground water contaminated with the organohalogen compounds. However, since the organohalogen compounds are hardly decomposable and a large amount of soil and ground water must be purified, any efficient and economical purifying techniques or methods have not been fully established.
For example, as the method of purifying soil contaminated with the organohalogen compounds, there are known a purifying method of using various catalysts; a method of absorbing and removing vapors of the organohalogen compounds by utilizing a volatility thereof; a thermal decomposition method of heat-treating excavated soil to convert the soil into harmless one; a method of purifying the soil by microorganisms; or the like. In addition, as to the ground water contaminated with the organohalogen compounds, there are known a method of extracting the contaminated ground water out of soil and converting the ground water into harmless one; a method of pumping the contaminated ground water and removing the organohalogen compounds therefrom; or the like.
Among these conventional methods of purifying soil or ground water contaminated with the organohalogen compounds, there have been proposed many methods of purifying the soil or ground water contaminated with the organohalogen compounds into harmless ones by mixing and contacting the soil or ground water with a purifying agent composed of iron-based particles.
For example, there is described a method of adding and mixing in soil, iron particles containing carbon in an amount of not less than 0.1% by weight to convert organohalogen compounds contained in the soil into harmless ones (Japanese Patent Application Laid-Open (KOKAI) No. 11-235577(1999)). Although in the method of the above Japanese Patent Application KOKAI, iron particles having specified specific surface area and particle size are used, since the particle size thereof is too large, it may be difficult to fully decompose the organohalogen compounds. Further, in the in-situ purification, the iron particles may fail to show a sufficient penetrability into soil.
There is described a soil-purifying agent composed of a water suspension containing iron particles having an average particle diameter of 1 to 500 μm (Japanese Patent Application Laid-Open (KOKAI) No. 2001-38341). However, since the iron particles used have a too large particle size, it may be difficult to fully decompose the organohalogen compounds. Further, in the in-situ purification, the iron particles have failed to show a sufficient penetrability into soil. Also, the soil-purifying agent contains, in addition to the iron particles, at least one of hydrophilic polymers, metal halides and inorganic carbonates. The hydrophilic polymers have a function for covering the iron particles to prevent contact between the iron particles and oxygen in air, whereas the metal halides have a function as a reducing agent (rust-preventive agent) for breaking immobilized oxidized portions of the iron particles, and the inorganic carbonates are used for the purpose of insolubilizing eluted Fe. Accordingly, the above Japanese Patent Application KOKAI is quite different in technical and inventive concept from the present invention.
There is described a method of using a water suspension containing spherical iron particles having an average particle diameter of less than 10 μm (Japanese Patent Application Laid-Open (KOKAI) No. 2001-198567). However, the water suspension containing the spherical iron particles is obtained by collecting dusts contained in waste gas produced during a refining process from an oxygen blowing converter for steel-making in which oxygen is blown into pig iron containing C, Si, P, etc., as impurities to refine the pig iron, and removing gases from the dusts. For this reason, it is suggested that the resultant spherical iron particles also contain impurities such as C, Si and P in the form of an oxide and, therefore, may fail to show a high purification ability to the organohalogen compounds.
There is described a method of purifying soil or ground water contaminated with organohalogen compounds using sulfur-containing iron particles (Japanese Patent Application Laid-Open (KOKAI) No. 2002-210452). However, since the iron particles thereof have a too large particle size, it may be difficult to fully reduce the organohalogen compounds. Further, since the iron particles used in the above method are obtained by atomizing a molten steel with water, it is suggested that the resultant iron particles usually contain a large amount of impurities derived from the molten steel. Therefore, the iron particles thereof may fail to show a high purification ability to the organohalogen compounds.
There is described a soil-purifying agent composed of a water suspension containing fine iron particles having an average particle diameter of 0.1 to 500 μm (Japanese Patent Application Laid-Open (KOKAI) No. 2002-282834). However, since the fine iron particles contain no sulfur, the resultant soil-purifying agent may fail to fully decompose and remove the organohalogen compounds. Further, the soil-purifying agent contains, in addition to the iron particles, at least one of amino acids, hydrophilic binders, metal halides and inorganic carbonates. The amino acids have a function of adsorbing the fine iron particles therein and facilitating the contact between the fine iron particles and contaminants such as organohalogen compounds, and the hydrophilic binders have a function of covering the fine iron particles to prevent the contact between the fine iron particles and oxygen in air. Whereas, the metal halides have a function as a reducing agent (rust-preventive agent) for breaking immobilized oxidized portions of the fine iron particles, and the inorganic carbonates are added for the purpose of insolubilizing the eluted Fe. Accordingly, the above Japanese Patent Application is quite different in technical and inventive concept from the present invention.
Also, there is described a method of using magnetite-containing iron composite particles for purifying soil or ground water contaminated with organohalogen compounds (Japanese Patent Application Laid-Open (KOKAI) No. 2002-317202). However, since the iron composite particles used contain no sulfur, it may be difficult to fully reduce the organohalogen compounds.
There is described a purification method using a water suspension containing metallic iron particles (U.S. Pat. No. 5,857,810). However, the metallic iron particles used in the above method have a particle size as large as 1 to 5 μm. Therefore, the metallic iron particles having such a large particle size may fail to fully decompose the organohalogen compounds. Further, in the in-situ purification, the metallic iron particles may fail to show a sufficient penetrability into soil.
There is described a soil-purifying agent composed of a water suspension containing iron particles having an average particle diameter of 1 to 200 μm (International Patent Application Laid-Open No. WO 01/008825). However, since the iron particles used have a too large particle size, it may be difficult to fully decompose the organohalogen compounds. Further, in the in-situ purification, the iron particles used may fail to show a sufficient penetrability into soil. Also, the soil-purifying agent contains, in addition to the iron particles, at least one of hydrophilic binders, metal halides and inorganic carbonates. The hydrophilic binders have a function of covering the iron particles to prevent the contact between the iron particles and oxygen in air, whereas the metal halides have a function as a reducing agent (rust-preventive agent) for breaking immobilized oxidized portions of the iron particles, and the inorganic carbonates are added for the purpose of insolubilizing eluted Fe. Accordingly, the invention of the above International Patent Application is quite different in technical and inventive concept from the present invention. Further, the water suspension containing the spherical iron particles is obtained by collecting dusts contained in waste gas produced during a refining process from an oxygen blowing converter for steel-making in which oxygen is blown into pig iron containing C, Si, P, etc., as impurities to refine the pig iron, and removing gases from the dusts. For this reason, it is suggested that the resultant spherical iron particles also contain impurities such as C, Si and P in the form of an oxide and, therefore, may fail to show a high purification ability to the organohalogen compounds.
There is described a soil-purifying agent composed of a water suspension containing nanometer-order metal particles produced from sodium boron hydride and iron chloride (International Patent Application Laid-Open No. WO 03/013252). However, in the above soil-purifying agent, boron derived from the raw material tends to remain in the particles and be eluted out therefrom. Therefore, the use of the soil-purifying agent tends to have problems concerning safety. Further, it is required to adhere noble metals such as Pd and Pt onto a surface layer of the nanometer-order metal particles in order to improve a purification ability thereof, so that the resultant soil-purifying agent tends to become expensive.
There are described the methods for purifying soil or ground water contaminated with organohalogen compounds using a composite of magnetite and iron (Japanese Patent Application Laid-open Nos. 2003-230876, 2004-058051, 2004-082102 and 2004-83086). However, in these methods, since the use of polyaspartic acid or salt thereof is not taken into consideration, the purifying agent may fail to fully penetrate into soil depending upon configurations of the soil.
As a result of the present inventors' earnest studies for solving the above problems, it has been found that the water suspension obtained by heat-reducing goethite particles having specific average major axis diameter, Al content and S content or hematite particles having specific average major axis diameter, Al content and S content, at a temperature of 350 to 600° C. to produce iron particles; forming a surface oxidation layer at the surface of the iron particles; adding an aqueous solution containing a specific amount of polyaspartic acid or salt thereof to the obtained iron composite particles; and mixing and stirring the mixture, is unexpectedly capable of treating organohalogen compounds and/or heavy metals contained in soil or ground water in an efficient, continuous and economical manner. The present invention has been attained on the basis of this finding.